Ultrafast laser flash photolysis (lambda(ex) = 375 nm) of para-methoxy-3-phenyl-3-methyl diazirine (p-CH3OC6H4CN2CH3) produced a transient absorption band in the 400-700 nm region. The carrier of the transient absorption is assigned to the S-1 electronic excited state of this compound based on quantum chemical calculations. The strongest vibrational mode of this S-1 excited state, predicted by RI-CC2/TZVP calculations, was directly observed in the mid-infrared region and had the same lifetime as the transient absorption band detected in the visible region, confirming that the same species is responsible for both transient spectra. The S-1 state undergoes solvation within 20 ps after its formation in polar solvents. Decay of the S-1 state leads to the formation of the isomeric diazo compound and singlet carbene. With 270 nm excitation, both singlet carbene and diazo compound are formed in a much more rapid process from the initially populated diazirine S-2 state (<4 ps), in competition with internal conversion to the S-1 state. The ultrafast spectroscopy and quantum calculations presented in this study provide a rather complete and consistent understanding of the structures and the decay kinetics of the excited states of an aryldiazirine and provide some conclusive answers to the pending general mechanistic questions concerning the photoisomerization of diazirine into diazo compound and the denitrogenation into carbenes.